This invention relates to methods and apparatus for detecting electrical defects in a semiconductor device or test structure having a plurality of features that are specifically designed to produce varying voltage potentials during a voltage contrast inspection. More particularly, it relates to voltage contrast techniques for detecting open and short type defects within the features of the circuit or test structure.
A voltage contrast inspection of a test structure is accomplished with a scanning electron microscope. The voltage contrast technique operates on the basis that potential differences in the various locations of a sample under examination cause differences in secondary electron emission intensities when the sample is the target of an electron beam. The potential state of the scanned area is acquired as a voltage contrast image such that a low potential portion of, for example, a wiring pattern might be displayed as bright (intensity of the secondary electron emission is high) and a high potential portion might be displayed as dark (lower intensity secondary electron emission). Alternatively, the system may be configured such that a low potential portion might be displayed as dark and a high potential portion might be displayed as bright.
A secondary electron detector is used to measure the intensity of the secondary electron emission that originates from the path swept by the scanning electron beam. Images may then be generated from these electron emissions. A defective portion can be identified from the potential state or appearance of the portion under inspection. The portion under inspection is typically designed to produce a particular potential and resulting brightness level in an image during the voltage contrast test. Hence, when the scanned portion""s potential and resulting image appearance differs significantly from the expected result, the scanned portion is classified a defect.
Several inventive test structures designed by the present assignee are disclosed in copending U.S. patent application Ser. No. 09/648,093 (Attorney Docket No. KLA1P016A) by Satya et al., filed Aug. 25, 2000, which application is incorporated herein in its entirety. In one embodiment, a test structure is designed to have alternating high and low potential conductive lines during a voltage contrast inspection. In one inspection application, the low potential lines are at ground potential, while the high potential lines are at a floating potential. However, if a line that is meant to remain floating shorts to an adjacent grounded line, both lines will now produce a low potential during a voltage contrast inspection. If there is an open defect present within a line that is supposedly coupled to ground, this open will cause a portion of the line to be left at a floating potential to thereby produce a high potential during the voltage contrast inspection. Both open and short defects causes two adjacent lines to have a same potential during the voltage inspection.
In theory, defects within the above described test structure may be found by comparing images of portions of the test structure that are designed to have identical appearances during voltage contrast inspection. Thus, any difference between two supposedly identical imaged portions can be classified as an electrical defect. However, certain physical defects within the conductive line may introduce xe2x80x9cnuisancexe2x80x9d defects into the inspection process. That is, physical defects may be inadvertently counted as electrical defects. In one voltage contrast technique, a first set of imaged adjacent conductive lines are subtracted from a second set of imaged adjacent lines. The imaged sets are selected so that they will be identical if they contain no defects. Thus, if the subtraction operation results in a nonzero value, imaged sets are flagged as having one or more defects. Unfortunately, this subtraction procedure may result in physical defects being flagged, as well as electrical defects.
The total defect count which results from a voltage contrast inspection on specially designed test chips can run into the thousands for a complete wafer. Manually reviewing and classifying these defects to isolate the electrical defects from the physical defects is very time consuming and tedious. Additionally, manual classification is inherently subject to human errors.
Accordingly, there is a need for improved apparatus and methods for detecting electrical defects in semiconductor device and test structures without having to manually filter physical defects from the inspection results.
Accordingly, mechanisms are provided for automatically filtering out physical defects from electrical defects during a voltage contrast inspection of a test structure on a semiconductor device. In general terms, the test structure is designed to include a plurality of features that will charge to specific voltage potentials when scanned with an electron beam during a voltage contrast inspection. Images of the scanned features are generated, and the relative brightness level of each feature depends on the corresponding potential of each feature during the inspection. That is, some features are expected to appear dark, and other features are expected to appear bright. If there is no defect present in the scanned feature, the corresponding image will have the expected number of bright and dark features. However, if there is a defect present, the number of dark and bright features within the generated image will not match expected results.
The test structure may also be designed so have substantially identical pairs of features. In one implementation, each pair includes a feature that is expected to appear bright and another feature that is expected to appear dark within the generated image. Thus, an image may be generated for each set of features. The generated images are then subtracted from each other to determine whether any defects are present. Alternatively, an image of an inspected set of features may be compared to a reference image constructed from a design database. A subtraction that results in a defect is then analyzed to determine whether it is an electrical defect or a physical defect.
In one embodiment, the aspect ratio of each scanned feature is known. If the defect has substantially the same aspect ratio as the known aspect ratio, it is determined to be an electrical defect. Otherwise, it is determined to be a physical defect. Other mechanisms are described for locating the found electrical defect and filtering out the location of physical defects during this process. Additionally, mechanisms are described for calibrating the inspection process based on a test structure having a plurality of known defects.
In one embodiment, a method of detecting defects in a plurality of features on a semiconductor device is disclosed. Each of the features is designed to have a particular potential when scanned with an electron beam. A first and second image of a first and second set of feature portions, respectively, are provided. The first and second images are generated as a result of scanning an electron beam over the first and second set of feature portions, and the first set of feature portions are designed to be substantially identical to the second set of feature portions when there is no defect present. The first image is subtracted from a second image to generate a difference image. When the difference image has a significantly sized defect that represents a difference between the first set of feature portions and the second set of feature portions, it is determined whether the defect is an electrical defect or a physical defect. In one implementation, this determination is accomplished by determining that the defect indicates an electrical defect when the defect has about a same aspect ratio as a one of the scanned feature portions. When the defect has a different aspect ratio than a one of the scanned feature portions, it is determined that the defect indicates a physical defect.
In a specific implementation, the features include a plurality of conductive lines, and the conductive lines are designed to include a plurality of lines at a grounded potential interleaved and are arranged parallel with a plurality of lines at a floating potential. An electrical defect is found when two adjacent conductive lines have a same potential when scanned with the electron beam. In another aspect, the scanned feature portions are a plurality of end stub portions of the conductive lines, and each end stub portion has a substantially same aspect ratio.
In a further implementation, it is determined that the electrical defect is a short when the when the defect has a first brightness value. It is determined that the electrical defect is an open when the differ when the defect has a second brightness value. In one aspect, the first brightness value is higher than the second brightness value. In yet another embodiment, when an electrical defect is found, the electron beam is then scanned along the defective feature perpendicular to the first scan to form a plurality of images of portions of the defective feature not scanned in the first scan and one or more features adjacent to the defective feature. A first image of the defective feature is subtracted from a second image of the defective feature to obtain a difference image. The defective feature is designed to have a first portion having a bright appearance and a second portion having a dark appearance when there is an open defect present within the defective feature. When the difference image has a significantly sized second defect that represents a difference between the first and second image of the defective feature, it is then determined whether the second defect represents a location of the electrical defect or a physical defect.
In a preferred implementation, determining whether the second defect represents a location of the electrical defect or a physical defect is accomplished by determining that the second defect represents a location of the electrical defect and that the electrical defect is an open when the second defect would be adjacent to a dark and bright portion of the defective feature if the difference image were overlaid with the first or second image. It is determined that the second defect represents a location of the electrical defect and that the electrical defect is a short defect when the second defect would not be adjacent to a dark and bright portion of the defective feature and the second defect touches two adjacent features if the difference image were overlaid with the first or second image. It is determined that the second defect represents a location of a physical defect when the second defect would not be adjacent to a dark and bright portion of the defective feature and the second defect does not touch two adjacent features if the difference image were overlaid with the first or second image.
In another aspect, the invention pertains to a computer program product for detecting defects in a plurality of features on a semiconductor device. Each of the features is designed to have a particular potential when scanned with an electron beam, the computer program product includes at least one computer readable medium and computer program instructions stored within the at least one computer readable product configured to cause the inspection system to perform one or more of the above described methods.
In another aspect, the invention pertains to an inspection system for detecting defects in a plurality of features on a semiconductor device. Each of the features is designed to have a particular potential when scanned with an electron beam. The system includes a beam generator for generating an electron beam, a detector for detecting electrons, and a controller arranged to cause the beam generator to scan an electron beam over a first and a second set of feature portions. The first set is designed be substantially identical to the second set of feature portions when there is no defect present. The controller is further arranged to generate a first image of the first set of feature portions and a second image of the second set of feature portions from electrons detected by the detector emitted from the scanned feature portions in response to the scanned electron beam. The controller is also configured to subtract the first image from the second image to generate a difference image.
When the difference image has a significantly sized defect that represents a difference between the first set of feature portions and the second set of feature portions and the defect has about a same aspect ratio as a one of the scanned feature portions, the controller is further configured to determine that the defect indicates an electrical defect within the scanned features. When the difference image has a significantly sized defect that represents a difference between the first set of feature portions and the second set of feature portions and the defect has a different aspect ratio than a one of the scanned feature portions, the controller is further configured to determine that the defect indicates a physical defect.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.